A crystalline polypropylene is excellent in rigidity, heat resistance, surface glossiness, etc., and however, there is a problem that the polypropylene is poor in impact resistance.
On that account, there have heretofore been proposed various methods to improve the impact resistance of such a polypropylene, for example, there is known a method in which crystalline polypropylene is incorporated with modifiers such as polyethylene, rubbery substance, etc. As such a rubbery substance (rubber component), an amorphous or a low crystalline ethylene/propylene random copolymer (EPR), polyisobutylene, polybutadiene, etc. are generally used.
In order to improve the impact strength of polypropylene by incorporation of such a rubbery substance into the polypropylene, however, a large amount of the rubbery substance must be contained in the polypropylene. Although the polypropylene composition thus contained a large amount of the rubbery substance is certainly improved in impact resistance, there was such problem that the composition largely deteriorates in rigidity, heat resistance and surface hardness.
Under such circumstances, there have been proposed various polypropylene compositions which contain the rubbery substance as mentioned above together with inorganic fillers such as talc in order to impart rigidity to the compositions (see, for example, Japanese Patent L-O-P Publns. Nos. 58459/85, 60154/85, 233048/86, 36348/86, 235350/87, 122751/88, 150343/88, 149845/89, 204947/89 and 271450/89).
Such a polypropylene composition containing a large amount of the rubbery substance, however, has a limit in improving their rigidity by means of incorporation thereinto of the inorganic filler, and hence there was such a problem that it is difficult to use the composition for the purposes where a high rigidity of the composition is required.
Therefore, it has been desired to develop a polypropylene composition excellent not only in rigidity and heat resistance but also in impact resistance and toughness.
Researches conducted by the present inventors on the polypropylene composition as referred to above resulted in the finding that in the case of conventionally known polypropylene compositions formed from polypropylene and an ethylene/propylene random copolymer (rubber component), a part of the amorphous component in the polypropylene is mingled in the rubber component, and this rubber component is bound by the amorphous component of the polypropylene, with the result that no sufficient effect of the rubber component for improving the impact resistance can be exhibited.
Then, the present inventors have found that in order to obtain a polypropylene composition excellent not only in rigidity and heat resistance but also in impact resistance, it is necessary to form a polymer alloy (polypropylene composition) having a molecular composite-like structure in which the rubbery substance (rubber component) is used as a matrix and polypropylene (crystal component) is used as a dispersion phase, differing from a polymer alloy formed from polypropylene and the rubbery substance which has a generally-known structure as mentioned above. Further, the present inventors have found that when a polypropylene composition has a structure in which the polypropylene microcrystals have a specific dimension in the directions of a-, b- and c-axes and the rubber component is present around the polypropylene microcrystals as a continuous phase having a specific average thickness, such a polypropylene composition is excellent in rigidity and heat resistance and also in impact resistance. Based on these findings, the present invention has been accomplished.
Incidentally, there has been made an attempt to obtain a polypropylene composition excellent in impact resistance by mixing polypropylene with other ethylene/.alpha.-olefin copolymers in place of the aforementioned amorphous or low crystalline ethylene/propylene random copolymers used as the rubbery substance. For instance, Japanese Patent Publication Nos. 25693/83 and 38459/83 disclose a composition containing a crystalline polypropylene and an ethylene/1-butene copolymers where the amount of the units derived from 1-butene is not more than 15 mol %, and also Japanese Patent L-O-P Publn. No. 243842/86 discloses a polypropylene composition containing a crystalline polypropylene and an ethylene/1-butene copolymer obtained by the use of a titanium-containing inhomogeneous catalyst system. The polypropylene compositions disclosed in these patent publications have certainly been improved in impact resistance and rigidity, however, their further improvement in impact resistance and toughness at low temperatures are desired.
Further, Japanese Patent Publication No. 42929/88 discloses a polypropylene composition containing a crystalline polypropylene and an ethylene/1-butene copolymer where the amount of the units derived from 1-butene is 25-10% by weight and the intrinsic viscosity [.eta.] of the copolymer is not more than 1.5 dl/g, however, the disclosed polypropylene composition fails to exhibit sufficient impact resistance.
Furthermore, Japanese Patent L-O-P Publn. No. 250040/91 discloses the use of an ethylene/1-butene block copolymer having 10-90% by weight of the units derived from 1-butene as an impact resistance improver for polypropylene, however, this ethylene/1-butene block copolymer is poor in compatibility with polypropylene and the effect of improving impact resistance as expected from the improver is insufficient.
The present invention has been made in view of the foregoing prior arts, and an object of the invention is to provide a polypropylene composition which is excellent in impact resistance and toughness as well as in rigidity and heat resistance, and also excellent in impact resistance at low temperatures.